For some it looks like an old hunk of junk best left to the scrap heap, but for collectors this old racing bike is bordering on Holy Grail status, meaning it should easily sell for more than $100,000 when it goes to auction this month.

Reason is it's one of the world's rarest Harley-Davidson motorcycles - a circa-1927 FHA 8-Valve V-Twin racer with a scramble-type sidecar, reputedly one of fewer than 50 examples built by the American manufacturer between 1916 and 1928.

This "barn find" old Harley was apparently delivered new through Millege Brothers of Melbourne and has been locked away in dry storage for much of the past 80 years.

1927 Harley-Davidson 8-valve racer up for auction. Photo: Contributed

Its original unrestored condition - plus it being one of only a handful of 8-valve Harley racers worldwide to survive - makes it incredibly sought after.

When these bikes were sold new they cost a substantial US$1500 to keep them out of the reach of private enthusiasts and only offered to racers of the times.

This particular bike is believed to have last raced some time in the 1930s, competing in Victorian dirt track events.

It goes to auction at the Shannons Melbourne Spring Auction on September 21 with no reserve.

Shannons expects the 8-valve racer to sell for "a substantial six-figure sum" based on pre-auction interest.

With the Australian dollar on the slide there'll be particular interest from international collectors, meaning Australia may lose one of its most historical motorcycles to an overseas buyer.